1. Field of the invention
This invention relates to light interference filters for lamps, and is directed more particularly to a method for making tantala/silica interference filters on the surfaces of tungsten-halogen incandescent lamps having molybdenum lead wires.
2. Description of the Prior Art
Thin film optical coatings, known as interference filters, which comprise alternating layers of two or more materials of different indices of refraction, are well known to those skilled in the art. Such coatings, or films, are used to selectively reflect or transmit light radiation from various portions of the electromagnetic radiation spectrum, such as ultraviolet, visible and infrared radiation. The films or coatings are used in the lamp industry to coat reflectors and lamp envelopes. One application in which the thin film optical coatings are useful is to improve the illumination efficiency, or efficacy, of incandescent lamps by reflecting infrared energy emitted by a filament, or arc, back to the filament or arc while transmitting the visible light portion of the electromagnetic spectrum emitted by the filament. This lowers the amount of electrical energy required to be supplied to the filament to maintain its operating temperature. In other lamp applications, where it is desired to transmit infrared radiation, such filters reflect the shorter wavelength portions of the spectrum, such as ultraviolet and visible light portions emitted by the filament or arc, and transmit primarily the infrared portion in order to provide heat radiation with little or no visible light radiation. An application of this latter type includes a typical radiant heater, wherein visible radiation emitted by the heater is unwanted.
Such interference filters useful for applications where the filter will be exposed to high temperature in excess of 500.degree. C., or so, have been made of alternating layers of tantala (tantalum pentoxide, Ta.sub.2 O.sub.5) and silica (SiO.sub.2) wherein the silica is the low refractive index material and the tantala is the high refractive index material. Such filters, and lamps employing same, are disclosed in U.S. Pat. Nos. 4,588,923; 4,663,557 and 4,689,519. In such lamp applications, the interference filters, which are applied on the outside surface of the vitreous lamp envelope containing the filament within, often reach operating temperatures of about 800.degree. C. These interference filters, or coatings, have been applied primarily using evaporation or sputtering techniques which, while capable of producing a satisfactory interference filter, have limitations with respect to difficulty in applying a uniform coating to any but a flat surface. Tubing used for making lamps, must be rotated in the sputtering or vacuum evaporation chamber as the coating is being applied. This technique does not lend itself to the application of uniform coatings, and is rather costly.
In U.S. Pat. No. 4,949,005, issued Aug. 14, 1990, in the name of Thomas G. Parham, et al, there is described a method for the manufacture of thin film interference filters consisting of alternating layers of tantala and silica suitable for high temperature use on electric lamps. Depending upon the individual layer thicknesses, such filters may be designed to reflect light with wavelengths falling within a particular range, while transmitting light of other wavelengths. As described in the '005 patent, one use for such thin film interference filters is as coatings on vitreous envelopes of incandescent lamps, which coatings improve lamp efficiency by reflecting infrared energy emitted by the lamp filament back onto the filament, while transmitting visible light emitted by the filament. The method for the manufacture of such multilayer coatings described in '005 patent essentially involves depositing alternating layers of tantala and silica upon the surface of the lamp by low pressure chemical vapor deposition. In order to avoid the development of catastrophic stresses when the coated lamps are subsequently burned, leading to poor adhesion and poor optical properties, the coated lamps are heat treated to a temperature at least as high as the temperature of the lamp surface when the lamp is burned. Moreover, during this heat treatment process, the temperature of the coated lamp is maintained between 550.degree. and 675.degree. C. for a period of time ranging between 0.5 hour and 5 hours before being exposed to the higher lamp burning temperature, to control the rate of formation and growth of tantala crystallites during the heat treatment. The higher temperature is applied for 0.1-5 hours, and is at least as high as the lamp surface when the lamp is burned. During the heat treatment process, a pattern of fine randomly oriented cracks develops, resulting in a decrease in the overall, or average, stress. Random cracking is a natural consequence of high stresses in thin films. The heat treatment allows cracked coatings to remain stable during lamp operation.
However, a particularly serious problem arises during heat treatment of the aforesaid filters on tungsten-halogen lamps. The external electrical current leads of such lamps typically are of molybdenum wire, the wires being molded to small pieces of molybdenum foil hermetically sealed and embedded within a pressed seal portion of the lamp. Because molybdenum is an easily oxidized metal, it tends to react with oxygen contained in the heat-treatment atmosphere. Volatile molybdenum oxides are formed on the lead wires, reducing the lead wire diameter and allowing oxygen to diffuse through the pressed seal, weakening or destroying the hermeticity of the seal. Accordingly, from the standpoint of lead wire and pressed seal integrity, the tantala-silica multilayer filter should be heat treated in an atmosphere of inert gas containing little or no oxygen.
The use of a heat-treatment atmosphere consisting of an inert gas, such as nitrogen or argon, with little or no oxygen content, results in a coating which, upon inspection, appears brown due to the absorption of visible light. This broad-band visible absorption is believed to result primarily from the pyrolysis of organic residues originating from the organometallic precursors used in the low pressure chemical vapor deposition multilayer process. If the heat-treatment atmosphere contains a significant amount of oxygen (&gt;2%, by volume), these trapped organic residues are apparently oxidized and eliminated via diffusion, producing heat-treated coatings which absorb very little of the incident visible light.
There exists, then, a problem in the heat treatment of typical tungsten-halogen lamps with envelopes coated with tantala/silica multilayer interference filters applied according to the method of Parham, et al. In particular, coatings designed to transmit visible light must be heat treated to approximately 800.degree. C. in an atmosphere containing at least 2% oxygen in order to produce thermally stabilized coatings with low absorption coefficients for visible light. On the other hand, heat-treatment atmospheres containing little or no oxygen must be used in order to avoid massive oxidation of the molybdenum current leads and, ultimately, destruction of the hermetic pressed-glass seals.
There is thus a need for an improved method for making a thin film interference filter on the surface of tungsten-halogen lamps, which method will permit heat treatment of the filter to temperatures of around 800.degree. C., without coloration of the filter and without significant oxidation of the molybdenum lead wires.